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<title>Alternatives to Wrapper #ifndef (The C Preprocessor)</title>

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<h3 class="section" id="Alternatives-to-Wrapper-_0023ifndef-1"><span>2.5 Alternatives to Wrapper #ifndef<a class="copiable-link" href="#Alternatives-to-Wrapper-_0023ifndef-1"> &para;</a></span></h3>

<p>CPP supports two more ways of indicating that a header file should be
read only once.  Neither one is as portable as a wrapper &lsquo;<samp class="samp">#ifndef</samp>&rsquo;
and we recommend you do not use them in new programs, with the caveat
that &lsquo;<samp class="samp">#import</samp>&rsquo; is standard practice in Objective-C.
</p>
<a class="index-entry-id" id="index-_0023import"></a>
<p>CPP supports a variant of &lsquo;<samp class="samp">#include</samp>&rsquo; called &lsquo;<samp class="samp">#import</samp>&rsquo; which
includes a file, but does so at most once.  If you use &lsquo;<samp class="samp">#import</samp>&rsquo;
instead of &lsquo;<samp class="samp">#include</samp>&rsquo;, then you don&rsquo;t need the conditionals
inside the header file to prevent multiple inclusion of the contents.
&lsquo;<samp class="samp">#import</samp>&rsquo; is standard in Objective-C, but is considered a
deprecated extension in C and C++.
</p>
<p>&lsquo;<samp class="samp">#import</samp>&rsquo; is not a well designed feature.  It requires the users of
a header file to know that it should only be included once.  It is much
better for the header file&rsquo;s implementor to write the file so that users
don&rsquo;t need to know this.  Using a wrapper &lsquo;<samp class="samp">#ifndef</samp>&rsquo; accomplishes
this goal.
</p>
<p>In the present implementation, a single use of &lsquo;<samp class="samp">#import</samp>&rsquo; will
prevent the file from ever being read again, by either &lsquo;<samp class="samp">#import</samp>&rsquo; or
&lsquo;<samp class="samp">#include</samp>&rsquo;.  You should not rely on this; do not use both
&lsquo;<samp class="samp">#import</samp>&rsquo; and &lsquo;<samp class="samp">#include</samp>&rsquo; to refer to the same header file.
</p>
<p>Another way to prevent a header file from being included more than once
is with the &lsquo;<samp class="samp">#pragma once</samp>&rsquo; directive (see <a class="pxref" href="Pragmas.html">Pragmas</a>).  
&lsquo;<samp class="samp">#pragma once</samp>&rsquo; does not have the problems that &lsquo;<samp class="samp">#import</samp>&rsquo; does,
but it is not recognized by all preprocessors, so you cannot rely on it
in a portable program.
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